Re: PPS in GPSD vs Chrony

[Gary E. Miller]

Yo Dennis!

On Tue, 30 Jun 2020 10:44:05 +0800
Dennis Law <cmlaw1993@gmail.com> wrote:

> We have actually been using the SHM0 output from GPSD "locked" with
> the PPS clock in Chrony.
> As per our current understanding, SHM0 takes the time directly from
> the NMEA without consideration for PPS.

Yes, which means it can easily be off a second.

> On the other hand, the SHM1 or Sock output from GPSD already has the
> full complete timestamp with PPS considerations.

Yup.

> Therefore, is it correct to say that the SHM0 locked with PPS in
> Chrony is somewhat similar to the SHM1 or Sock output from GPSD?

Nope.  That is backwardsd   The SHM1 from gpsd is locked by gpsd to
SHM0.

> If both of these mechanisms are indeed similar, are there any
> performance benefits of one over the other?

SHM0 +/- 300 ms.  SNM1 +/- 1 us

> We had a look at the PPS code in GPSD and Chrony and found the GPSD's
> version to be a lot more advanced (eg. handling of different PPS
> frequencies).

Yes.  But rarely used.

> Therefore, we are wondering perhaps using GPSD's PPS would have some
> sort of performance benefit over Chrony's PPS.

Unlikely.  Only way to know is to test.

> Also, the reason we are asking is because we are trying to achieve a
> high accuracy of 0.1ms in our system clock by using both GPSD +
> Chrony.

NMEA will never get to 0.1ms.

RGDS
GARY
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Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        gem@rellim.com  Tel:+1 541 382 8588

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